Hydroxyapatite (HAp) is the major inorganic component of bone and dental tissues, but other crystalline calcium phosphates may coexist. In addition, other such phases may be involved in dissolution and reprecipitation processes, assuming pH and concentration gradients. Given recent results for HAp, precise but markedly different from erratic bulk-solubility results, the solubility isotherms of such other calcium phosphates need to be (re)determined and their relationship to that of HAp investigated. Objectives: To ascertain the solubility isotherm and equilibrium solid composition and constitution for adding tetracalcium phosphate (TTCP; Ca₄(PO₄)₂O) and dicalcium phosphate dihydrate (brushite, DCPD; CaHPO₄.2H₂O) to 100 mM KCl solution at 37̊C. Methods: Solid titration, using laser-scattering end-point detection, was used over the pH ranges (DCDP) 3.2 - 11.6 and (TTCP) 3.3 - 9.2. Precipitates just past the end-point were characterized by XRD (crystallography), EDX (Ca/P ratio), SEM and TEM (morphology and surface conditions). Results: For TTCP, the stable phase in contact with the solution at equilibrium at pH 3.6 and 4.5 was a calcium-deficient HAp: Ca/P ~1.48 and ~1.62 respectively, the expected DCPD did not appear at pH 3.6. Brushite was more soluble in contact with the solution than HAp over the pH range 3.2 - 11.6, although it has been reported to be more stable below pH 4.2. Conclusion: Calcium-deficient HAp was found as the stable contact phase at pH 3.6 and 4.5, consistent with previously-reported behaviour. Hence, HAp appears to be the stable phase for solution Ca/P values of 1.0 to 2.0, for the pH range of dental relevance, in 100 mM KCl. The failure to form brushite is noteworthy as hitherto this has been expected at pH < 4.2. Other calcium phosphates, such as tricalcium phosphate (TCP) and octacalcium phosphate (OCP), and Ca/P values outside the range 1 - 2, need to be examined.

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